Method of improving electrical characteristics of a liquid phase deposited silicon dioxide film by plasma treatment

ABSTRACT

A silicon dioxide layer grown by liquid phase deposition is, subjected to an oxygen or hydrogen plasma treatment to enhance the physical and electrical properties thereof. The plasma treatment is carried out at a temperature of about 300° C.

FIELD OF THE INVENTION

The present invention relates to a method of improving the properties ofa liquid phase deposited silicon dioxide layer, and in particular to amethod of improving the dielectric properties of a liquid phasedeposited silicon dioxide layer.

BACKGROUND OF THE INVENTION

SiO₂ layer grown by liquid phase deposition (LPD) at room temperaturenot only has a low residual stress but also is advantageous in thefabrication of various semiconductor devices which requires a lowfabrication temperature, such as the fabrication of LCD and3-dimensional IC devices.

In U.S. Pat. No. 4,468,420, H. Kawahara et al. first disclose a methodfor making a silicon dioxide coating which comprises dipping a substratein a mixed solution obtained by adding boric acid to an aqueous silicondioxide-saturated solution of hydrofluosilicic acid. One of the presentinventors, Ching-Fa Yeh, and his co-workers, in their article, entitled"Performance and Off-State Current Mechanisms of Low-TemperatureProcessed Polysilicon Thin Film Transistors with Liquid Phase DepositedSiO₂ Gate Insulator", C.F. Yeh, S.S. Lin, T.Z. Yang, C.L. Chen and Y.C.Yang, IEEE Trans. on Electron Devices, Vol. 41, pp. 173-179, 1994, havesuccessfully used a SiO₂ grown by this liquid phase deposition (LPD)technique as a gate dielectric in a poly-Si thin film transistor whichshows satisfactory electrical characteristics. However, the breakdownfield strength of the LPD-SiO₂ film is about 7-8 MV/cm and the interfacetrap density at the LPD-SiO₂ /Si interface is about 5×10¹¹ eV⁻¹ cm⁻²,which still can be improved in comparison with a thermal SiO₂ film.

S. Yoshitomi, S. Tomlika and N. Honeji in their article, entitled "TheCharacteristics of Si MOS Diodes Using the SiO₂ Films Prepared by theLiquid Phase Deposition", International. EDMS, pp. 22˜25, 1992, disclosea thermal annealing process to enhance the properties of the SiO₂ filmsgrown by the liquid phase deposition, in which annealing in O₂ at atemperature of 400° C. is found to be effective to improve thecharacteristics of the LPD-SiO₂ film.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to provide a lowtemperature method to enhance the dielectric characteristics of theLPD-SiO₂ film without losing the merit of low-temperature process ofLPD.

In order to accomplish the aforesaid objective, a method of improvingthe properties of liquid phase deposited SiO₂ film provided according tothe present invention comprises subjecting a liquid phase deposited SiO₂film to an oxygen or hydrogen plasma treatment, preferably an oxygenplasma treatment.

Preferably, the oxygen and hydrogen plasma treatments are carried out ata pressure of 0.1-0.25 torr and a temperature of room temperature to450° C.

Preferably, the plasma treatment is carried out with a power density of0.56-0.85 W/cm² for a period of 1-3 hours.

The present method can effectively improve the breakdown field strengthand interface trap density of LPD SiO₂ film without losing the merit oflow-temperature process of LPD.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot which shows the distribution of interface trapdensities at LPD SiO₂ /Si interface of MOS diodes, wherein curve (a)represents the MOS diode containing a LPD SiO₂ film without anytreatment; (b) represents the MOS diode containing a LPD SiO₂ filmsubjected to an O₂ thermal annealing treatment at 600° C.; (c)represents the MOS diode containing a LPD-SiO₂ film subjected to a H₂/N₂ plasma treatment at 300° C.; and (d) represents the MOS diodecontaining a LPD-SiO₂ film subjected to an O₂ plasma treatment at 300°C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a method of improving thecharacteristics of an liquid phase deposited SiO₂ (LPD-SiO₂) film by anannealing treatment, which comprises subjecting the LPD-SiO₂ film to anO₂ or H₂ /N₂ plasma treatment. In one of the preferred embodiments ofthe present invention, the breakdown field strength of an LPD-SiO₂ filmincreases from 8.3 MV/cm to 9.8 MV/cm and the interface trap densitythereof reduces from 5×10¹¹ eV⁻¹ cm⁻² to 2×10¹⁰ eV⁻¹ cm⁻² after beingtreated by the present method. The breakdown field strength and theinterface trap density of an LPD-SiO₂ film before or after the treatmentwere measured on a MOS diode which was fabricated without post-metalanneal.

The invention will be further illustrated by the following examples. Thefollowing examples are only meant to illustrate the invention, but notto limit it.

EXAMPLES 1-2

35 g of silica powder having a purity of 99.99% was added to 1 liter of4M hydrofluosilicic acid (H₂ SiF₆) to obtain a silica-saturated H₂ SiF₆solution. Furthermore, to supersaturate the resulting silica-saturatedH₂ SiF₆ solution with SiO₂, 2.63 ml of water and 13.5 ml of 0.1M boricacid were added to 50 ml of the resulting silica-saturated H₂ SiF₆solution.

A silicon substrate cleaned by standard procedures was immersed in theSiO₂ -supersaturated H₂ SiF₆ solution at a temperature of 30° C. for onehour. On the surfaces of the silicon substrate SiO₂ films having athickness of about 1500 Å were formed.

The silicon substrate was then placed in a parallel plate plasma reactorto undergo hydrogen and oxygen plasma treatments respectively accordingto the conditions listed as follows:

    ______________________________________                                        Ex. 1. Hydrogen plasma                                                                       H.sub.2 /N.sub.2 : 60 sccm/60 sccm, 0.25 torr                  Ex. 2. Oxygen plasma                                                                         O.sub.2 : 150 sccm, 0.25 torr                                  Frequency      13.56 MHz                                                      Temperature    300° C.                                                 Power density  0.7 W/cm.sup.2                                                 Time           1 hour                                                         ______________________________________                                    

On the surface of one of the plasma-treated SiO₂ films located at theupper side of the silicon substrate an aluminum layer was formed bythermal evaporation. Aluminum electrodes of various areas were thenformed by lithography.

Prior to removing the SiO₂ film located at the bottom side of thesilicon substrate by etching, a protective photo-resist layer was formedon the aluminum electrodes. A bottom electrode layer was then formed bydepositing an aluminum layer on the bottom surface of the siliconsubstrate. AMOS diode was obtained after the protective photo-resistlayer was stripped.

Control Example 1:

The procedures of Example 2 were repeated except that the O₂ plasmatreatment was not used.

Control Example 2:

The procedures of Example 2 were repeated except that the O₂ plasmatreatment was replaced by an O₂ thermal annealing treatment which wascarried out at 600 ° C. in accordance with the method disclosed by S.Yoshitomi, S. Tomilka and N. Honeji, International. EDMS, PP. 22˜25,1992.

The SiO₂ films were characterized by their refractive indexes andetching rates wherein the former were measured by using an ellipsometerand the latter was measured at room temperature by using a P-etchsolution of 48% HF:70% HNO₃ :H₂ O=3:2:60. The results are shown in Table1.

                  TABLE 1                                                         ______________________________________                                                                 Refractive                                                                              P-etch rate,                                        LPD-SiO.sub.2 film                                                                            index, n  Å/sec                                  ______________________________________                                        Control Ex. 1                                                                          without treatment                                                                             1.43      20                                         Control Ex. 2                                                                          O.sub.2 thermal annealing                                                                     1.432     7.6                                                 (600° C.)                                                     Ex. 1    H.sub.2 /N.sub.2 plasma (300° C.)                                                      1.45      9.2                                        Ex. 2    O.sub.2 plasma (300° C.)                                                               1.456     7.5                                        ______________________________________                                    

The data in Table 1 show that the refractive indexes of the LPD-SiO₂films modified by the present method (Examples 1 and 2) are increasedfrom the original refractive index of 1.43 (Control Example 1) to valueswhich are very close to that of a thermal SiO₂ film (1.46). The P-etchrate of the originally formed LPD-SiO₂ film is 20 Å/sec (ControlExample 1) which is remarkably reduced to 9.2 and 7.5 Å/sec after beingsubjected to hydrogen plasma treatment (Example 1 ) and oxygen plasmatreatment (Example 2), respectively. We believe that the hydrogen andoxygen plasma treatments can render the LPD-SiO₂ film much denser (lessporous), which results in a higher refractive index and a lower P-etchrate.

Table 2 shows the distribution of breakdown electric field strength(E_(BD)) of MOS diodes fabricated in the Examples 1-2 and ControlExamples 1-2. It can be seen from Table 2 that the LPD-SiO₂ filmstreated by hydrogen and oxygen plasma (Examples 1 and 2) have a betterperformance than the thermal annealed LPD-SiO₂ film (Control Example 2)in terms of the distribution of breakdown electric field strength of MOSdiode. In particular, the LPD-SiO₂ film treated by oxygen plasma(Example 2) has the maximum E_(BD) (10 MV/cm) and a failure percentagewhich is not higher than 10% until 9 MV/cm electric field strength. Thismay be due to the more denser (less porous) structure of the plasmatreated LPD-SiO₂ film.

                  TABLE 2                                                         ______________________________________                                        Failure, %                                                                                      Control Ex. 2                                                                            Ex. 1 (H.sub.2 /N.sub.2                                                                Ex. 2 (O.sub.2                          E.sub.BD,                                                                            Control Ex. 1                                                                            (thermal   plasma   plasma                                  MV/cm  (untreated)                                                                              annealed)  treated) treated)                                ______________________________________                                        1      10         5          5        3                                       2      3          3          3        --                                      3      --         --         3        --                                      4      --         3          --       --                                      6      3          --         --       --                                      7      5          --         --       --                                      8      35         15         2        2                                       9      44         66         25       5                                       10     --         8          62       79                                      11     --         --         --       11                                      ______________________________________                                    

The Terman method [E. H. Nicollian and J. R. Brews, MOS (Metal OxideSemiconductor) Physics and Technology. New York: Wiley, P. 325, 1982]was used to calculate a typical distribution of interface trap densityfor MOS diodes fabricated in the Examples 1-2 and Control Examples 1-2,and the results are shown in FIG. 1. It can be seen from FIG. 1 that, atenergy level of 0 eV, the interface trap density at LPD-SiO₂ /Siinterface is reduced from 5×10¹¹ eV⁻¹ cm⁻² [Curve (a)] to about 2×10¹⁰eV⁻¹ cm⁻ 2 after the LPD-SiO₂ film being subjected to the hydrogenplasma [Curve (C)] and the oxygen plasma treatment [Curve (D)]. It isbelieved that the significant reduction of interface trap density byhydrogen and oxygen plasma treatments is because that hydrogen andoxygen diffuse to the interface of LPD-SiO₂ /Si with the assistance ofplasma, and thus diminish the interface defects.

The embodiments of the present invention described above are to beregarded in all respects as being merely illustrative and notrestrictive. Accordingly, the present invention may be embodied in otherspecific forms without deviating from the spirit thereof. The presentinvention is therefore to be limited only by the scopes of the followingappended claims.

What is claimed is:
 1. A method of improving electrical characteristicsof a liquid phase deposited silicon dioxide film comprising forming aliquid phase deposited silicon dioxide film on a substrate andsubjecting the liquid phase deposited silicon dioxide film to an oxygenor hydrogen plasma treatment.
 2. The method according to claim 1,wherein the liquid phase deposited silicon dioxide film is subjected toan oxygen plasma treatment.
 3. The method according to claim 1, whereinthe plasma treatment is carried out at a pressure of 0.1-0.25 torr and atemperature of about 300° C.
 4. The method according to claim 1, whereinthe plasma treatment is carried out with a power density of 0.56-0.85W/cm² for a period of 1-3 hours.